The 1st, 2, and 3 paradigms in the database (I did not sleep well yesterday, because the mosquito ~~)

When I woke up in the middle of the night, a mosquito was buzzing next to me. I didn't die after N times, but I had a lot of spam marks on my body. It was so depressed that it was dawn...

1. 1nf)
In any relational database, the first paradigm (1nf) is the basic requirement for the relational model. databases that do not meet the first paradigm (1nf) are not relational databases.
The first paradigm (1nf) means that each column in the database table is an inseparable basic data item. The same Column cannot contain multiple values, that is, an attribute in an object cannot have multiple values or duplicate properties. If duplicate attributes exist, you may need to define a new object. A new object consists of duplicate attributes. The new object has one-to-multiple relationships with the original object. In the first paradigm (1nf), each row of the table contains only information of one instance. In short, the first paradigm is a non-repeated column.

2 second Paradigm (2nf)
The second Paradigm (2nf) is established on the basis of the first paradigm (1nf), that is, to satisfy the second Paradigm (2nf) must satisfy the first paradigm (1nf) first ). The second Paradigm (2nf) requires that each instance or row in the database table be able to be uniquely divided. To implement differentiation, you usually need to add a column to the table to store the unique identifier of each instance. This unique attribute column is called as the primary keyword, primary key, and primary code.
The second Paradigm (2nf) requires that the attributes of an object fully depend on the primary keyword. The so-called full dependency refers to the fact that there cannot be an attribute that only depends on a part of the primary keyword. If so, this attribute and this part of the primary keyword should be separated to form a new entity, the relationship between the new object and the original object is one-to-multiple. To implement differentiation, you usually need to add a column to the table to store the unique identifier of each instance. In short, the second paradigm is that non-primary attributes are not partially dependent on primary keywords.

All database tables with single keywords comply with the second paradigm, because it is impossible to have a combination of keywords.

3. Third Paradigm (3nf)
The third paradigm (3nf) must satisfy the second Paradigm (2nf) first ). In short, the third paradigm (3nf) requires that a database table do not contain information about non-primary keywords already contained in other tables. In short, the third paradigm is that attributes do not depend on other non-primary attributes.